(1) Field of the Invention
The present invention relates to a vertical magnetic recording medium and a process for the preparation thereof. More specifically, it relates to a vertical magnetic recording medium comprising a substrate and a magnetic layer formed thereon, said magnetic layer consisting of columnar structures and voids and being composed substantially of a metal selected from cobalt and iron, and an oxide thereof, and a process for preparing this vertical magnetic recording medium by the vacuum deposition method.
(2) Description of the Related Art
A film of a cobalt type alloy such as a Co-Cr alloy has been used as a thin film type vertical magnetic recording medium. This cobalt type alloy film is ordinarily formed by the sputtering method or the electron beam vacuum deposition method. However, formation of a cobalt type alloy thin film by the sputtering method is not suitable for the industrial production because the film-forming speed is low. The electron beam vacuum deposition using cobalt and chromium is defective in that control of the composition of the film is very difficult because cobalt and chromium are greatly different from each other in the melting point and the vapor pressure. Moreover, each of the foregoing conventional processes has a problem such that in order to improve the magnetic characteristics in the vertical direction, the substrate must be heated at a temperature of about 150.degree. to about 300.degree. C. during the formation of a film.
As means for overcoming these disadvantages, there has been proposed a process in which cobalt is used and electron beam deposition is carried out in oxygen-introduced vacuum, whereby there is formed a cobalt type vertical magnetic recording medium having a two-phase mixed state, which includes cobalt particles and non-ferromagnetic cobalt monoxide particles (see Collection of Lectures at 7th Meeting of Japan Applied Magnetism Association, 7aA-9 to 7aA-B, November 1983).
However, this process for forming a vertical magnetic recording medium by electron beam deposition of cobalt in oxygen-introduced vacuum is still not satisfactory in the following points.
Since only an oxygen gas is introduced, an oxygen gas-getting action is caused by a cobalt deposited onto the substrate or the inner wall of the vacuum chamber at the vacuum deposition step, and the pressure of the vacuum chamber is drastically changed. Accordingly, there is no reproducibility in the filmforming conditions or the magnetic characteristics of the magnetic layer. Furthermore, when the magnetic layer is continuously formed on a continuous substrate composed of an organic polymer film, the magnetic characteristics and the thickness of the magnetic layer become uneven in the machine direction of the substrate. Moreover, cracks are readily formed on the surface of the magnetic layer composed of cobalt and cobalt oxide, which is formed on the continuous substrate. These cracks cause bit error and drop-out at the time of high-density recording. Therefore, formation of such cracks should be avoided.
European Patent No. 116,881, which was laid open after the convention priority date of the present application, proposes a process for forming a vertical magnetic recording medium composed mainly of iron oxide by the opposed target sputtering method in which iron or iron oxide is used as the starting material and argonoxygen gas is introduced or by the vacuum deposition method using an ion gun. Since the magnetic layer formed by this process is composed mainly of iron oxide, the magnetic layer is fragile and poor in the flexibility and is easily worn.
Furthermore, when a tape-like magnetic recording medium is prepared on an industrial scale by forming a magnetic layer continuously on a polymeric substrate, if the sputtering method is adopted, the film-forming speed is still insufficient and the manufacturing cost is increased. Furthermore, when there is adopted a vacuum deposition method in which iron is evaporated while introducing an oxygen ion or activated oxygen, the structure of the apparatus placed in the vacuum chamber becomes complicated and the number of control factors is increased. Therefore, the process is not preferred from the industrial viewpoint.